When traveling abroad, a subscriber of a mobile phone is reachable on the Home PLMN's (Public Land Mobile Network) (HPLMN) MSISDN (Mobile Subscriber ISDN (Integrated Services Digital Network) Number). A call to the MSISDN leads to the establishment of a roaming call leg, from GMSC (Gateway Mobile Switching Center) in HPLMN to Visited MSC (VMSC) in (foreign) Visited PLMN (VPLMN). When the call to the MSISDN is established from the VPLMN where the subscriber currently resides, then the call is established as a ‘tromboning call’. That is to say, the call is established from the VPLMN where the call originates, to the HPLMN of the called subscriber and from the HPLMN back to the VPLMN where the called party currently resides. This is a direct result of the principles of ‘roaming’, as currently applied in the GSM/3G mobile network.
The ‘tromboning’ aspect of establishing a terminating call towards the roaming subscriber, when the call is established from the same visited country, is generally considered a drawback associated with the contemporary GSM/3G mobile network. The implication is namely that (a) the calling party pays international call rate, for the call from VPLMN to HPLMN and (b) the called party pays for the international roaming call leg, for the establishment of the call from HPLMN to VPLMN. Hence, even though calling and called party currently reside in the same country, they both pay international call charge.
Basic Optimal Routing (BOR) has been specified by ETSI (European Telecommunications Standards Institute) as a potential remedy for this solution. BOR entails that the MSC in the visited country acts as GMSC and interrogates the HLR (Home Location Register) in the HPLMN of the called subscriber. If the called subscriber is located in the same country as the MSC where the call is established, then the HLR may provide a roaming number to the GMSC, which has the effect that the call is established locally.
BOR is commercially not implemented. Put differently, although BOR is formally specified, it has limitations that hamper commercial deployment of it. For example, agreement is needed between HPLMN and VPLMN, since BOR requires coordinated action by the two networks. In addition, BOR provides no clear guideline how the charging of the call shall be done. Interaction between BOR and Call forwarding is a further area that leads to unforeseen, non-transparent call cases. In addition, terminating call handling is often subject to special handling in the GMSC, such as prepaid, personal greeting service or Free-divert-to-voicemail. Such service invocation can't be handled by a GMSC in a network other than the called party's HPLMN. Hence, BOR is in practice not a suitable solution.
Another solution would be the use of a local subscription, i.e. buying a local SIM (Subscriber Identity Module) card. Although the subscriber is now reachable through a local number, for which local call rate applies for the calling party and a zero call rate for the called party, the subscriber is no longer reachable under his/her home MSISDN. In addition, purchasing a local SIM card has the hassle of swapping cards in the phone and safely storing a subscriber's home SIM card. This is especially disadvantageous considering the fact that much data as well as applications can be stored on the SIM card. That data will become inaccessible. SIM swap is, for the issue at hand, considered not a useful solution. Of course, one may purchase a local phone+SIM and keep the home SIM card in the home mobile phone. This method is considered equally unpractical, for reason of SIM card stored data, as described, and the hassle of having two phones to keep on you. In addition, this has the disadvantage that data downloaded/stored/processed on one phone is not available on the other phone.
As an example publication U.S. Pat. No. 7,912,464 is presented, disclosing a solution whereby the calling party can be reached on a locally-routable-international number. For example, a subscriber with MSISDN +31 6 516 xxx who is traveling to Singapore may be called locally on the number *31 6 516 xxx. Such service may e.g. be offered by Singtel and works only when the called subscriber is currently registered in an MSC from Singtel. The leading * (or other designated character combination, as decided by the operator) serves as an indication to the MSC where the call is established that the call shall be routed locally. The solution entails that the MSC routes the call to a special service platform, which obtains a roaming number in the MSC where the called party is currently registered. This solution has the disadvantage that the calling party has to use special character combination to establish the call. So, it is still not transparent for the calling party.
In certain circumstances, the called subscriber may explicitly wish to be reachable on a local number, without using a different phone and without using a different SIM card. One prominent example is the case that the subscriber resides for a long(er) duration in the visited country for business or other purpose. So, (s)he wants to allow others (business relations) to reach him/her on a local number. However, the subscriber wants to remain reachable on his/her home MSISDN as well. The currently known solutions don't offer this capability, as they have one or more of the following disadvantages:                The subscriber has to use another, locally purchased/rented terminal;        The subscriber has to use a different SIM card;        The subscriber can't provide his/her local contacts a local phone number;        The subscriber is not reachable on his/her home MSISDN;        The calling party has to dial a special prefix in front of the number.        
The concept of Multiple subscriber profile (MSP; 3GPP TS 23.097) is known from the 3G network. MSP will, however, not resolve the problem at hand. MSP is strictly associated with one particular operator. An MSP subscriber may receive multiple MSISDNs of one operator, but when the subscriber is roaming abroad, (s)he will be reachable under his/her home MSISDN only. MSP has no relation with a visited network and does not have the capability to allow the subscriber to be reachable under a local MSISDN.
The Gateway Location Register (GLR; 3GPP TS 23.119) allows for using a ‘Location register’ in the visited network. The GLR acts as an ‘HLR Proxy’ in the visited network. As such, it keeps a local copy of the subscriber data from the home network. The GLR has, however, no relation to the visited network HLR. The GLR can't be used for assigning a local MSISDN to the subscriber. The subscription profile stored in the GLR in the visited network is still a copy of the Home subscriber profile, i.e. no relation to the visited network.
The concept of Alternative Roaming Provider (ARP), which forms part of EU Roaming Regulation III (EU Regulation 531/2012) stipulates that a roaming subscriber should have the possibility to be served (specifically: charged) by a different service provider, namely the ARP, than his home PLMN provider when roaming abroad (in EU). When served by an ARP, a user still has the same service profile from the home network and the same MSISDN. RR III relates only to charging of communication sessions when abroad.
A further possibility would be to use ‘multi-SIM’. A UICC (Universal Integrated Circuit Card) may be equipped with multiple SIM/USIM applications. When switching on the phone, the user gets the option presented to select the SIM/USIM (Universal SIM) from which he/she wants to start the phone. However when you have a UICC with a SIM/USIM of a local network, there is the dilemma that you're not reachable on your home MSISDN.
Accordingly, a need exists to avoid the above-mentioned drawbacks and to provide a possibility for a roaming subscriber to be reached locally in the visited network while maintaining the option to be reached via the home subscription in an easy way.